Winding with cooling ducts

ABSTRACT

Electrical inductive apparatus including a winding having a plurality of pancake-type coils disposed in a fluid-filled tank. Large radii edge shields formed of woven conductive strands disposed on an insulating core are placed about the inner and outer edges of the pancake coils, and secured in position by insulating channel members. Insulating washer members and spacer blocks provide cooling ducts adjacent the major surfaces of the pancake coils. In one embodiment of the invention, the cooling ducts have entrance and exit openings formed by spaced inward steps in the edges of the leg portion of the insulating channel members and in the edges of the adjacent insulating washer members.

United States Patent 1,424,016 7/1922 Haworth 2,724,735 11/1955 Johnston Inventors Appl. No,

Filed Patented Assignee Harold R. Moore;

I-Iarral T. Robin, both of Muncie, Ind.

53,968 July 10, I970 Aug. 31, I971 WINDING WITH COOLING DUCTS 10 Claims, 4 Drawing Figs.

US. Cl

Int. CL

References Cited UNITED STATES PATENTS 3,376,530 4/1968 Fisher .Q.

Primary Examiner- Thomas J. Kozma Attorneys-A. T. Stratton, F. E. Browder and Donald R.

Lackey ABSTRACT: Electrical inductive apparatus including a winding having a plurality of pancake-type coils disposed in a fluidfilled tank. Large radii edge shields formed of woven conductive strands disposed on an insulating core are placed about the inner and outer edges of the pancake coils, and secured in position by insulating channel members. Insulating washer members and spacer blocks provide cooling ducts adjacent the major surfaces of the pancake coils. In one embodiment of the invention, the cooling ducts have entrance and exit openings formed by spaced inward steps in the edges of the leg portion of the insulating channel members and in the edges of the adjacent insulating washer members.

WINDING WITH COOLING DUCTS BACKGROUND OF THE INVENTION l. Field of the Invention The invention relates in general to electrical inductive apparatus, such as power transformers, and more specifically to electrical winding structures for such apparatus.

'2. Description of the Prior Art The heat generated in the windings of electrical inductive apparatus, such as power transformers, must be quickly and efficiently removed to prevent the winding temperature from exceeding a predetermined maximum magnitude, determined by the type of electrical insulation utilized. In the prior art, cellulosic insulating materials, such as pressboard and crepe paper, are commonly used for the solid insulation required in the transformer, and a fluid insulating and cooling dielectric, such as mineral oil, is used to impregnate the solid insulation and to carry heat away from the winding in cooling ducts designed for this purpose. The mineral oil may circulate through the cooling ducts due to the natural thermal siphon effect, or it may be forced through the cooling ducts by suita ble pumping means. The heated oil is directed to external heat exchangers and the cooled oil is then circulated back through the winding. The electrical stresses at the innerand outer edges of the pancake coils, and the cooling duct arrangement between the pancake coils, are interrelated. If the electrical gradient is high at the edges of the pancake coil, it deleteriously affects the construction of the cooling ducts, as high electrical stresses in the cooling ducts must be avoided in order to prevent ionization-of the oil. The problem is compounded by the fact that electrical stress is transferred from the solid insulation to the 'oil due to their different dielectric constants. High stresses at the edges of the pancake coils require that the duct width be reduced, which deleteriously affects ,the cooling of the coil surfaces, and it complicates the construction of the ducts,

which have to be designed to obtain a smooth transition from solid to liquid insulation, which often leads to increasing the axial length of the coil stack, compared to a construction where high electrical stresses adjacent the coil edges is not a problem. Thus, the size, weight and cost of the apparatus is deleteriously affected by high electrical stresses at the edges of the pancake coils.

Edge shields of the prior art, used to shield the inner and outer edges of the pancake'coils, are commonly formed of a solid, rectangular metallic strap, similar to the conductors of which the pancake coil is formed. These conductors have rounded corners, such as 0.03l-inch or 0.047-inch comer radii, but with the trend to a higher voltages, these prior art edge shields possess certain disadvantages. For example, if the width of the edge shield is increased to the dimension required to reduce the potential gradient below the corona inception level, the eddy and circulating currents induced into the shield conductor by the stray or leakage flux of the transformer, causes excessive heating of the shield and its surrounding insulation. If the width of the edge shield is reduced to bring the losses therein within an acceptable level, the potential gradient at the edge shields may be sufficient to cause ionization of the oil in the adjacent cooling ducts.

Therefore, it would be desirable to provide new and improved electrical inductive apparatus which includes a winding structure having a plurality of pancake-type coils, and a new and improved edge shield and cooling duct arrangement which substantially reduces the potential gradient adjacent the edges of the coils and simplifies the cooling duct structure, without loss of cooling efficiency.

SUMMARY OFTI-IE INVENTION Briefly, the present invention is new and improved electrical inductive apparatus having at least one electrical winding constructed of pancake-type coils, disposed in a fluid-filled tank. Edge shields are provided for the inner and outer edges of the pancake coils, and insulating channel members secure the edge shields in the desired position. Insulating washer members with spacer blocks are disposed between the coils to provide cooling ducts for circulation of the fluid adjacent the major surfaces of the pancake coils.

The edge shields are formed of a plurality of metallic strands which are braided or woven together about an insulating core, which allows the diameter of the shield to be increased to the desired large dimensions, without-excessive heating due to eddy and circulating currents. The use of the large radii edge shield lowers the potential gradient adjacent the edges of the pancake coils, and simplifies the duct structure, enabling entry and exit openings to the cooling ducts to be provided at the upper and lower ends of the coils, by providing inward steps in the leg portions of the insulating channel members, or by cooperative inward steps in both the insulating channel members and the adjacent edges of the insulating washer members.

The dimensions of the edge-shielding means may even be increased such that they overhang the sides of their associated pancake coils, without disadvantage. In this particular embodiment, spacer members are provided between the leg portions of the insulating channel members and the adjacent surfaces of the pancake coils, to provide support in the space created by the overhanging edge shields, and at the same time removing the blanketing effect of the edge shields by enabling cooling fluid to circulate between the leg portions of the insulating channel members and the adjacent portions of the pan cake coil. 7

The disclosed arrangements for providing entry and exit openings to the cooling ducts eliminates the need for placing spacer members between the leg portions of the insulating channel members and the adjoining washer members, which provides an overall reduction in the axial. dimensions of the coil stack, compared with prior art constructions, thereby reducing the size, weight and cost of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be more readily understood when considered in view of the following detailed description of exemplary embodiments thereof, taken with the accompanying drawings, in which:

FIG. I is an elevational view, partially cut away and partially in section, illustrating a transformer which utilizes the teachings of the invention;

FIG. 2 is a diagrammatic view of an electrical winding of the type which may utilizethe teachings of the invention;

FIG. 3 is a fragmentary, perspective view of an electrical winding having openings to a cooling duct, with the openings being constructed according to an embodiment of the invention; and

FIG. 4 is a fragmentary, perspective view of an electrical winding having openings to a cooling duct, with the openings being constructed according to another embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. I in particular, there is shown an elevational view, partially cut away, in sec tion, of electrical inductive apparatus 10, such as a power transformer. Inductive apparatus 10 includes a magnetic corewinding assembly 12 disposed in a tank or casing 14, which is shown partially cut away. Tank 14 may be filled to a predetermined level with a suitable insulating and cooling dielectric fluid, such as oil. As is customary in transformers of the shellform-type, the magnetic core-winding assembly 12 includes first and second magnetic core sections 16 and 18, respectively, each formed of a plurality :of stacked metallic magnetic laminations 20, arranged to form at least one opening for receiving electrical coils, and with the magnetic core sections being disposed in side-by-siderelation to form a common winding leg portion 22. Inductive apparatus 10 may be of the single, or of the polyphase type, and of the isolated winding, or of the autotransformer type. The magnetic core-winding assembly 12 also includes a static plate and a winding constructed of a stack of pancake-type coils, such as static plate 23 and winding 25 having pancake coils 24, 26, 28 and 30, shown in the fragmentary cross-sectional view in FIG. 2, all of which are arranged on a common centerline 32 and connected to form the winding, or windings of the electrical apparatus. For example, the finish or outer turn of pancake coil is electrically connected to a line terminal 34 and to the static plate 23, and adjacent pancake coils across the winding are interconnected with start-start, finish-finish connections. Specifically, pancake coils 24 and 26 are interconnected with start-start connection 36 between their innermost turns, pancake coils 26 and 28 are interconnected with finish-finish connection 38 between their outermost turns, and pancake coils 28 and 30 are interconnected with start-start connection 40 between their innermost turns.

Inductive apparatus 10 shown in FIG. 1 has been sectioned adjacent pancake coil 28, as indicated by the line II in FIG. 2. The pancake coils of winding 25, such as pancake coil 28 shown in FIGS. 1 and 2, have a plurality of conductor turns 42, formed of a conductor having one or more insulated, axially spaced strands of electrically conductive material, such as copper or aluminum, which conductor is wound about a mandrel sized to provide an opening 44 for receiving leg portion 22 of the magnetic core. The turns 42 of the pancake coil define first and second major opposed surfaces 46 and 48, which are joined by the opening 44 or inside edge, and by the outside edge of the pancake coil. As illustrated in FIG. 2, certain of the pancake coils may be slightly dished to axially space the coils according to the magnitude of the potential difference between them.

In order to provide a rounded, equipotential surface adjacent the inner and outer edges of each pancake coil, such as pancake coil 28, to prevent electrical stresses from concen trating on these edges, and to reduce the magnitude of the electrical stresses in the cooling ducts, conductive edge strip or shielding means 50 is disposed in radially spaced relation about the outer edge, and a similar shielding means 52 is disposed about the inner edge. As will be hereinafter explained, each shielding means has an electrically conductive portion disposed about an insulating core, and this structure is wrapped with suitable electrical insulating means. The conductive portions of the outer and inner shielding means are electrically connected to the adjacent turn of the pancake coil, and they are held in position by a plurality of insulating channel members, such as by channel member 54 on the outer edge of pancake coil 28, and by insulating channel member 56 on the inner edge of pancake coil 28. Each of the insulating channel members include first and second leg portions connected by a back portion, and they may be formed of pressboard, or other suitable electrical insulating material.

As hereinbefore explained, the edge-shielding strips for pancake coils constructed according to the teachings of the prior art present certain manufacturing and electrical problems. When the edge-shielding means includes a conductive portion formed of a solid metal, such as copper, it is difficult to bend the edge strip to closely conform to the inner and outer edges of a pancake coil. This problem, along with the problem imposed by the limitation on the maximum diameter of the solid edge-shielding means due to its heating, complicates and increases the cost of insulating and cooling the windings of a power transformer, especially for those rated in the extra high voltage class (EHV).

These problems of the prior art are overcome, following the teachings of the invention, by forming the edge-shielding strips from' a flexible, braided conductor which is woven tightly about the outer surface of a ropelike insulating core. This construction of the edge-shielding means may be better understood by examining FIG. 3, which is a fragmentary, perspective view of the upper edge of pancake coil 28, and a portion of pancake coil 26, illustrating edge-shielding means 50.

Edge-shielding means 50 includes a flexible, braided or woven conductor 60 which snugly encompasses an insulating core 62, and this structure is surrounded with tight-fitting insulating means 64, such as paper tape. The insulating core 62 is formed of a material which will maintain the desired cross-sectional configuration, such as silicone rubber.

As hereinbefore stated, the braided conductor portion 60 is formed of a large plurality of relatively fine wires or strands 66. The portion of braided conductor 60 within circle 68 is shown magnified within circle 68' to illustrate a braided or woven construction which has been found to be suitable. It will be noted that the directions of the wires or strands 66 form a substantial angle with respect to the longitudinal dimension of the core 62, such as an angle of 60. Changes in the length of the edge-shielding means 50 may thus occur during the manufacturing processing steps of the pancake coils, without adversely affecting the conductive portion of the edge strip. In other words the braided or woven conductor is free to change its length, along with the change in the dimension of the core material, without pulling away or creating voids between the edge-shielding strip and the pancake coil..

While the diameter of the individual strands 66 is not critical, the diameter should be selected to be as small as practical. Very fine or small diameter strands provide a substantially smooth outer surface on the woven conductor 60, which is necessary in order to reduce the potential gradient at its surface below the corona inception level. Fine strands also add to the flexibility of the edge strip, facilitating its handling and bending. Further, fine strands are individually so flexible that any broken ends that may extend outwardly from the surface of the woven conductor will be bent inwardly to conform to the surface of the conductor when the braided conductor is subsequently insulated. This is important, as outwardly extending wires or strands may cause the formation of corona. Small diameter strands also aid the structure electrically, as reducing the diameter of the strands increases their resistance to the flow of electrical current. Small diameter strands also allow more strands to be utilized in the braid, which reduces the magnitude of circulating currents due to the larger number of contacts between the strands.

The braided conductor 60 is electrically insulated to a predetermined build or outer dimension. This may be easily accomplished by passing the braided conductor through a taping machine of the type used for taping or insulating solid electrical conductors. Insulating tape 64, which may be formed of paper, is wrapped about the braided conductor until the layers of paper tape reach the predetermined desired build dimension, as required by the specific application.

The diameter of the braided conductor, the diameter of the individual strands in the conductor, and the dimension to which the braided conductor is insulated, will depend upon the specific application, the voltage rating, and the basic insulation level (BIL) of the apparatus. For example, a braided conductor having a radius of 0.25 inch, formed of copper wires each having a diameter of 0.0063 inch, which is insulated with a plurality of layers of paper-insulating tape to a radius of 0.319 inch, will be used on apparatus tested at 1,800 kv. BIL.

While the braided conductor in this example was constructed of copper, it is to be understood that other metals such as aluminum, stainless steel and the like, may be used. When higher resistivity materials are used, the outside diameter of the braided conductor may be increased without suffering excessive losses due to eddy and circulating currents.

It should be noted that the conductor portion 60 of edge strip means 50 may be shaped to provide the most advantageous configuration in reducing the potential gradient adjacent the edges of the pancake coil. For example, it has been found that a semielliptical configuration is excellent, as it combines a large radii with a wide width dimension, while adding little to the radial dimensions of the shielding strip.

Pancake coil 28, as shown in the fragmentary, perspective view of FIG. 3, has two parallel-connected, axially spaced conductive strands per conductor turn, i.e.,' strands 70 and 72 with each of thestrands being electrically insulated with suitable insulating means 74, and then the two strands are held together with insulating means 76, to provide a composite conductive member which may be easily wound to form the pancake coil. It is to be understood, however, that the conductive member may have any desired number of conductive strands, in order to reduce the eddy current losses. The plura'lity of strands may be transposed, using transposing arrangements well known in the art, in order to reduce circulating currents between the parallel-connected strands. Regardless of the number of conductive strands per conductor turn, the conductive portion of the outer-inner edge-shielding strips are electrically connected to one of the strands of the immediately adjacent turn of the pancake coil, illustrated by the connection 80 in FIG. 3, which interconnects conductive strand 70 with the conductive portion 60 of the edge-shielding means 50. The semielliptical configuration of the conductive portion 6Q of the edge-shielding means 50 provides excellent shielding for both of the strands of the conductor of which the pancake coil 28 is wound.

The edge-shielding means are held in position about the inner and outer edges of the pancake coils by insulating channel members, such as by insulating channel member 82, shown in FIG. 3. Insulating channel member 82 has first and second spaced, substantially parallel leg portions 84 and 86, respectively, and a connecting back portion 88, with the opening provided by the spaced leg members being sized to snugly fit over the edge of the pancake coil and its edge-shielding means.

In order for the edge-shielding means to effectively shield the edges of the pancake coils, and reduce the potential gradient in the adjacent cooling ducts, it may be necessary to construct the edge-shielding means with a width dimension which is greater than the width of the coil edges it is to shield. This'embodiment of the invention is also shown in FIG. 3. Instead of this overhanging of the edge-shielding means, i.e., the extension of the edge-shielding means past the two major planes of the pancake coil, being a disadvantage, however, it is turned into an advantage by providing a plurality of spacer members, such as spacer 90, between theleg portions of the insulating channel members and the adjacent coil surfaces, which spacers support the leg portions of the channel members while still allowing access to this space by the cooling fluid. Thus, the blanketing effect by the insulating channel members, common in the prior art, is eliminated, without undue electrical stress in the oil space between the leg portions of the insulating channel members and the coil surfaces. Electrical stress is reduced in this space due to the large radii of the edge-shielding means surrounding the coil edges, and the relatively small width of this space.

As illustrated in FIG. 1, cooling ducts are formed adjacent the major surfaces of the pancake coils by disposing insulating washer members between adjacent pancake coils, such as insulating washer member 92 shown in FIGS. 1 and 3, which has a plurality of insulating blocks glued to both sides thereof, such as blocks 94 on one side thereof, and blocks 96 on the other. This construction is shown in FIG. 1, with the insulating washer-member 92 being partially cut away in order to illustrate the blocks 96 on its reverse side. Or, two insulating washer members each having blocks on only one side thereof, may be placed with their smooth sides together to form ducts adjacent the pancake coils.

In the prior art, the entrance and exit openings to the cooling ducts adjacent the major surfaces of the pancake coils, is usually accomplished by gluing spacer members on the leg portions of the insulating channel members. This practice, however, increases the axial length of the winding structure, and it would therefore be desirable to provide new and improved structures for forming the entrance and exit openings to the ducts, especially when the adjacent coils are close together, i.e., at the start-start and finish-finish connections of the winding.

for the winding stack than prior art structures which utilize such spacers. This embodiment enables the insulating washer. member between pancake coils to be tightly sandwiched between the insulating channel members of adjacent pancake coils. Specifically, the edges of the leg portion of the insulating channel members, and the edges of the insulating washer member, where entrance and exit openings are desired, are cooperatively stepped to provide the openings to the cooling ducts. As shown most clearly in FIG. 3, the legportion 86 of insulating channel member 82 has a plurality of spaced inward steps which provide a plurality of openings therein, such as openings and 102, and the adjacent edge of the insulating washer member 92 has a plurality of similarly spaced inward steps which provide a plurality of openings, such as openings 104 and 1 06. The inward steps in the leg portion of the insulating channel member 82 are aligned with the inward steps in the insulating washer member 92, to provide access openings to the cooling duct. For example, inward steps 100 and 104 are aligned to provide opening 108, and inward steps 102 and 106 are aligned to provide opening 110, which openings lead to the cooling duct disposed between pancake coil 28 and insulating washer member 92.

While the arrangement for providing exit and entrance openings to the cooling ducts shown in FIG. 3 is preferred, as the depth or length of the inward steps in the leg portions of the insulating channel members and in the insulating washer member need not be great, it is to be understood that entrance and exit openings may be provided by utilizing inward steps in only the leg portion of the insulating channel member, or in only the edges of the insulating washer member, but the depth or length of the steps will have to be increased. For example, FIG. 4 is a fragmentary, perspective view of pancake coil 28, similar to that shown in FIG. 3, except the entrance and exit openings are formed by extending the length of the steps I00 and 102 in the insulating channel members, such that the steps extend into the curved or connecting back portions of the channels, and eliminating the inward steps in the insulating washer member. Like reference numerals in FIGS. 3 and 4 indicate like components. Like reference numerals with a prime mark indicate like components, but modified according to the embodiment of the invention shown in FIG. 4.

In summary, there has been disclosed new and improved encased, fluid-cooled, electrical inductive apparatus having a winding formed of a plurality of pancake-type coils, and edgeshielding means disposed about the edges of the pancake coils which substantially reduces the potential gradient adjacent the edges of the pancake coils and in the adjacent cooling ducts. The edge shields are fastened in position by insulating channel members, and the coils are axially spaced, with insulating washer members and spacer blocks providing cooling ducts adjacent the major surfaces of the pancake coils. The edgeshielding means may be constructed with the desired radius, or radii, without regard to overheating due to stray flux, as it is formed of a plurality of electrically conductive strands which are woven or braided together about an insulating core. The braided edge-shielding strips about the edges of the pancake coils, in addition to being easily bent to conform to the edges ofthe coils, change their dimensions along with any changes in the dimensions of the pancake coils during the manufacturing process steps, to retain the desired tight fit between the edge strip shielding means and the pancake coils.

The use of the braided or woven electrical conductor in the edge-shielding means also overcomes prior art maximum size limitations on the diameter of the conductive portion of the shielding edge strip. This is important, as the prior art size limitation is a distinct disadvantage with the recent trend to higher voltages. The braided construction of the conductive portion of the edge strip, which includes a plurality of small diameter wires or conductive strands, effectively limits circulating currents due to the relatively high resistance of the small diameter strands to the flow of electrical current, and to the contact resistance between the large plurality of strands. Therefore, the size of the braided conductor may be increased to the diameter necessary to reduce the potential gradient at the surface of the conductor well below the corona inception level, without danger of overheating the insulation surrounding the edge strip shielding means due to the FR loss of the conductors. The reduction in the magnitude of the electrical stresses about the edges of the pancake coils enables new and improved structures to be used for providing entrance and exit openings to the cooling ducts adjacent the pancake coils, which structure utilizes a plurality of spaced inward steps in the leg portion of the insulating channel, which may provide the opening by itself, or in cooperation with a plurality of spaced inward steps in the insulating washer member.

We claim as our invention: 1. Electrical inductive apparatus, comprising: a tank, electrical winding means disposed in said tank, fluid-cooling means disposed in said tank, said electrical winding means including a plurality of axially spaced pancake-type coils, each having first and second major opposed outer surfaces, an opening which extends between its major surfaces, and inner and outer edges, means electrically connecting said plurality of pancake coils to provide at least one series path through said electrical winding means, insulating shielding means disposed about the inner and outer edges of at least certain of said pancake coils, insulating channel members having leg portions joined by a back portion, said insulating channel members being disposed about said insulated shielding means and the inner and outer edges of said pancake coils, insulating washer members disposed between said axially spaced pancake coils, and spacer means disposed between the insulating washer members and the major surfaces of adjacent pancake coils, to provide cooling ducts adjacent the major surfaces of said pancake coils, at least the leg portion of certain of said insulating channel members being stepped inwardly with spaced inward steps of a predetermined dimension, to provide inlet and outlet openings for the cooling ducts.

2. The electrical inductive apparatus of claim 1 wherein the edges of certain of the insulating washer members have a plurality of spaced inward steps therein, which are aligned with the spaced inward steps in the leg portions of the insulating channel members, to cooperatively provide inlet and outlet openings to the cooling ducts.

3. The electrical inductive apparatus of claim 1 wherein the insulated shielding means overhangs the edges of at least certain of the pancake coils by a predetermined first dimension, on each side thereof, providing a space between the leg portions of the insulating channel members and the adjacent major surfaces of their associated pancake coils, and including spacer members disposed in this space to provide support for the leg portions of the insulating channel members as well as cooling for the portion of the pancake coils disposed under the leg portions of the insulating channel members.

4. The electrical inductive apparatus of claim I wherein the insulated shielding means includes an electrically conductive portion having a cross-sectional configuration which is semielliptical.

5. The electrical inductive apparatus of claim 1 wherein the insulated shielding means includes an insulating core, an electrically conductive portion formed of a plurality of electrically conductive strands woven together about said core to provide a substantially smooth outer surface, and insulating means disposed about the electrically conductive portion to a predetermined build dimension.

6. The electrical inductive apparatus of claim I wherein the insulated shielding means is electrically connected to the adjacent turn of its associated pancake coil.

7. Electrical inductive apparatus, comprising:

a tank,

electrical winding means disposed in said tank,

fluid-cooling means disposed in said tank,

said electrical winding means including a plurality of axially spaced pancake-type coils, each having first and second major opposed outer surfaces, an opening which extends between the first and second major surfaces, and inner and outer edges,

means electrically interconnecting said plurality of pancake coils, with start-start, finish-finish connections,

insulating channel members having leg portions joined by a back portion, said insulating channel members being disposed about the inner and outer edges of said pancake coils,

insulating washer members disposed between said axially spaced pancake coils, and contacting the adjacent insulating channel members adjacent the start-start and finish-finish connections, spacer means disposed between said insulating washer members and the major surfaces of adjacent pancake coils providing cooling ducts for said fluid-cooling means,

the leg portions of certain of the insulating channel members being discontinuous, having a plurality of spaced, inward steps,

the outer edges of the insulating washer member adjacent the inward steps on the leg portions of the insulating washer members also having a plurality of spaced inward steps, which are aligned with the inward steps on the insulating channel member to provide entry and exit openings to the cooling ducts.

8. The electrical inductive apparatus of claim 7 including insulated shielding means disposed about the inner and outer edges of at least certain of the pancake coils, between the edges of the coils and the insulating channel members.

9. The electrical inductive apparatus of claim 8 wherein the insulated shielding means includes an electrically conductive portion formed of a plurality of electrically conductive strands woven together about an insulating core.

10. The electrical inductive apparatus of claim 8 wherein the insulated shielding means overhangs the edges of the pancake coils, providing a space between the leg portions of the insulating channel members and the adjacent major surfaces of its associated pancake coil, and including spacer means disposed in this space to provide support for the leg portions of the insulating channel members, and allow the fluid-cooling means access to the portion of the pancake coils disposed between the leg portions of the insulating channel members. 

1. Electrical inductive apparatus, comprising: a tank, electrical winding means disposed in said tank, fluid-cooling means disposed in said tank, said electrical winding means including a plurality of axially spaced pancake-type coils, each having first and second major opposed outer surfaces, an opening which extends between its major surfaces, and inner and outer edges, means electrically connecting said plurality of pancake coiLs to provide at least one series path through said electrical winding means, insulating shielding means disposed about the inner and outer edges of at least certain of said pancake coils, insulating channel members having leg portions joined by a back portion, said insulating channel members being disposed about said insulated shielding means and the inner and outer edges of said pancake coils, insulating washer members disposed between said axially spaced pancake coils, and spacer means disposed between the insulating washer members and the major surfaces of adjacent pancake coils, to provide cooling ducts adjacent the major surfaces of said pancake coils, at least the leg portion of certain of said insulating channel members being stepped inwardly with spaced inward steps of a predetermined dimension, to provide inlet and outlet openings for the cooling ducts.
 2. The electrical inductive apparatus of claim 1 wherein the edges of certain of the insulating washer members have a plurality of spaced inward steps therein, which are aligned with the spaced inward steps in the leg portions of the insulating channel members, to cooperatively provide inlet and outlet openings to the cooling ducts.
 3. The electrical inductive apparatus of claim 1 wherein the insulated shielding means overhangs the edges of at least certain of the pancake coils by a predetermined first dimension, on each side thereof, providing a space between the leg portions of the insulating channel members and the adjacent major surfaces of their associated pancake coils, and including spacer members disposed in this space to provide support for the leg portions of the insulating channel members as well as cooling for the portion of the pancake coils disposed under the leg portions of the insulating channel members.
 4. The electrical inductive apparatus of claim 1 wherein the insulated shielding means includes an electrically conductive portion having a cross-sectional configuration which is semielliptical.
 5. The electrical inductive apparatus of claim 1 wherein the insulated shielding means includes an insulating core, an electrically conductive portion formed of a plurality of electrically conductive strands woven together about said core to provide a substantially smooth outer surface, and insulating means disposed about the electrically conductive portion to a predetermined build dimension.
 6. The electrical inductive apparatus of claim 1 wherein the insulated shielding means is electrically connected to the adjacent turn of its associated pancake coil.
 7. Electrical inductive apparatus, comprising: a tank, electrical winding means disposed in said tank, fluid-cooling means disposed in said tank, said electrical winding means including a plurality of axially spaced pancake-type coils, each having first and second major opposed outer surfaces, an opening which extends between the first and second major surfaces, and inner and outer edges, means electrically interconnecting said plurality of pancake coils, with start-start, finish-finish connections, insulating channel members having leg portions joined by a back portion, said insulating channel members being disposed about the inner and outer edges of said pancake coils, insulating washer members disposed between said axially spaced pancake coils, and contacting the adjacent insulating channel members adjacent the start-start and finish-finish connections, spacer means disposed between said insulating washer members and the major surfaces of adjacent pancake coils providing cooling ducts for said fluid-cooling means, the leg portions of certain of the insulating channel members being discontinuous, having a plurality of spaced, inward steps, the outer edges of the insulating washer member adjacent the inward steps on the leg portions of the insulating washer members also having a plurality of spaced inward steps, which are aligned with the inward steps on the insulating chaNnel member to provide entry and exit openings to the cooling ducts.
 8. The electrical inductive apparatus of claim 7 including insulated shielding means disposed about the inner and outer edges of at least certain of the pancake coils, between the edges of the coils and the insulating channel members.
 9. The electrical inductive apparatus of claim 8 wherein the insulated shielding means includes an electrically conductive portion formed of a plurality of electrically conductive strands woven together about an insulating core.
 10. The electrical inductive apparatus of claim 8 wherein the insulated shielding means overhangs the edges of the pancake coils, providing a space between the leg portions of the insulating channel members and the adjacent major surfaces of its associated pancake coil, and including spacer means disposed in this space to provide support for the leg portions of the insulating channel members, and allow the fluid-cooling means access to the portion of the pancake coils disposed between the leg portions of the insulating channel members. 